1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with a boiling water reactor (BWR) fuel assembly having a one-way coolant flow valve at its bottom nozzle inlet which allows inflow of coolant but automatically closes upon flow reversal to prevent rapid depletion of fuel assembly coolant inventory in the event of a loss of coolant accident (LOCA).
2. Description of the Prior Art
Typically, large amounts of energy are released through nuclear fission in a nuclear reactor with the energy being dissipated as heat in the elongated fuel elements or rods of the reactor. The heat is commonly removed by passing a coolant in heat exchange relation to the fuel rods so that the heat can be extracted from the coolant to perform useful work.
In a nuclear reactor generally, a plurality of the fuel rods are grouped together to form a fuel assembly. A number of such fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self-sustained, nuclear fission reaction. The core is submersed in a flowing liquid, such as light water, that serves as the coolant for removing heat from the fuel rods and as a neutron moderator. Specifically, in a BWR the fuel assemblies are typically grouped in clusters of four with one control rod associated with each four assemblies. The control rod is insertable within the fuel assemblies for controlling the reactivity of the core. Each such cluster of four fuel assembies surrounding a control rod is commonly referred to as a fuel cell of the reactor core.
A typical BWR fuel assembly in the cluster is ordinarily formed by a N by N array of the elongated fuel rods. The bundle of fuel rods are supported in laterally spaced-apart relation and encircled by an outer tubular channel having a generally rectangular cross-section. The outer flow channel extends along substantially the entire length of the fuel assembly and interconnects a top nozzle with a bottom nozzle. The bottom nozzle fits into the reactor core support plate and serves as an inlet for coolant flow into the outer channel of the fuel assembly. Coolant enters through the bottom nozzle and thereafter flows along the fuel rods removing energy from their heated surfaces.
During LOCA incident (e.g. a recirculation line break), a break in the primary circuit causes coolant depletion from the reactor core. Coolant in each fuel assembly then reverses its upward flow direction and exits downward through the inlet of the fuel assembly bottom nozzle into the lower plenum of the core and then out of the primary circuit. This loss of coolant causes the fuel in the assembly to overheat (after occurrence of critical heat flux (CHF)) and possibly start to melt until the emergency core cooling systems (ECCS) get activated and provide sufficient heat transfer. In plants subject to such shortcoming in LOCA performance, NCR guidelines restrict operation of plants to remain at or below a certain power level. Such plants are often referred to as "LOCA limited."
Consequently, it is readily apparent that a need exists for some means to prevent short-term depletion of coolant from the fuel assemblies due to occurrence of a LOCA event so that efficient heat transfer can still proceed until activation of the ECCS. The implementation of such means would allow LOCA limited plants to be operated at higher power levels, resulting in significant gains in economy and safety.